High-rate and long-life performance of a truncated spinel cathode material with off-stoichiometric composition at elevated temperature

A truncated octahedral structure spinel cathode material with a nominal composition of Li_1.08Al_0.08Mn_1.85Co_0.05O_3.9F_0.1 (LAMCOF) and an Al-rich surface layer is successfully prepared by a sol-gel route combined with a temperature-controlled heat-treatment process. Concentration gradient of Mn⁴⁺ ions is found to decrease through the surface to the interior of the as-prepared sample. As the consequence, the LAMCOF sample delivers high rate capability and excellent cycling stability at elevated temperature, showing an initial discharge capacity of 111.1 mAh g⁻¹ and capacity retention of 70.5% over 850 cycles at 1C rate under 55 °C. Even at 5C rate under 55 °C, it also displays a high capacity of 102.5 mAh g⁻¹ with capacity retention of 80.0% over 850 cycles. These results reveal the importance of an off-stoichiometric strategy and a temperature-controlled heat-treatment process for the preparation of LiMn₂O₄-based spinel cathode material with unique crystal orientation and Mn⁴⁺-rich surface layer, which both decrease the Mn dissolution and provide interfacial stability while preserving fast Li⁺ diffusion, resulting in improvement of cycle performance and rate capability of the as-prepared material. This proposed synthesis method does not need a supplementary coating process and is likely suitable to prepare other high performance cathode materials.